(1) Field of the Invention
The present invention relates to a sintered body containing both cubic system boron nitride (hereinafter, referred to as CBN) and wurtzite-structured boron nitride (hereinafter, referred to as WBN) which can be used for cutting steel having a high hardness, and a method of producing the sintered body.
(2) Description of the Prior Art
CBN is commercially synthesized by a method, wherein low density boron nitride (hereinafter, referred to as gBN) is once melted in a melting medium, such as alkali metal or the like, and then CBN is precipitated under a static ultra-high pressure. In general, CBN is a powder formed from a single crystal, having a particle size of from 1 .mu.m to several hundreds of .mu.m and having a high strength and hardness, but also having has a cleavability. While, WBN is directly synthesized from gBN through phase conversion without the use of a melting medium by a method, wherein a pressure of about several hundreds Kb's generated by the explosion of an explosive is applied to the gBN, and polycrystal powders consisting of secondary particles having a particle size of from 0.1 .mu.m to several tens of .mu.m, are formed from aggregated primary particles having a size of several tens of nm, and have a high hardness and strength, and has no cleavability.
When a sintered body for a cutting tool is produced by containing CBN or WBN having the above described properties in the sintered body, the resulting sintered body exhibits the characteristic properties of the respective powders. In general, a sintered body containing CBN is sharp but results in a roughly cut surface due to the reason that the CBN particles have a shape having an acute angle. Further, the sintered body is apt to crack easily due to the cleavability of the CBN particles. While, a sintered body containing WBN is somewhat inferior in the sharpness to a sintered body containing CBN due to the irregular-shaped polycrystals of the WBN particles, the surface roughness of a material cut by the sintered body is lower than the surface roughness of a material cut by the sintered body containing CBN. Furthermore, the sintered body rarely cracks due to the absence of cleavability of the WBN particles.
In order to compensate the drawbacks of CBN and WBN, sintered bodies containing both CBN and WBN have been proposed in Japanese Patent Laid-Open Specification Nos. 77,359/81 and 97,448/80. Japanese Patent Laid-Open Specification No. 97,448/80 discloses a sintered body containing both WBN and CBN, which sintered body is produced from a starting material mixture of WBN, metal and ceramic by converting a part of the WBN into CBN during the sintering. While, Japanese Patent Laid-Open Specification No. 77,359/81 discloses a sintered body produced by sintering a starting material mixture of CBN, WBN, metal and ceramic, and consisting of 15-60% by volume of high density boron nitride and the remainder being cermet (metal and ceramic), which high density boron nitride consists of 4-16% by volume of CBN and 96-84% by volume of WBN.
Both of these inventions do not refer to the relation between the particle sizes of CBN and WBN.
The sintered bodies disclosed in these prior arts can be easily produced and are excellent in the cutting performance, wear resistance and workability, but are sometimes broken by chilled roll, ultra-heat resisting alloy, cast iron and the like, and are still insufficient in the cutting performance and wear resistance.
The inventors have made various investigations for developing a high density boron nitride-containing sintered body having a property adapted for cutting ultra-heat resisting alloy, cast iron and the like, and have discovered that when the high density boron nitride to be added to a starting material mixture for sintering is limited to two kinds of CBN and WBN, and further the ratio of the amount of CBN to that of WBN and the ratio of the average particle size of CBN to that of WBN are limited within certain ranges, a sintered body having excellent properties for cutting the above described materials can be obtained.